// Map is a concurrent map with amortized-constant-time loads, stores, and deletes. // It is safe for multiple goroutines to call a Map's methods concurrently. // // It is optimized for use in concurrent loops with keys that are // stable over time, and either few steady-state stores, or stores // localized to one goroutine per key. // // For use cases that do not share these attributes, it will likely have // comparable or worse performance and worse type safety than an ordinary // map paired with a read-write mutex. // // The zero Map is valid and empty. // // A Map must not be copied after first use.
// read contains the portion of the map's contents that are safe for // concurrent access (with or without mu held). // // The read field itself is always safe to load, but must only be stored with // mu held. // // Entries stored in read may be updated concurrently without mu, but updating // a previously-expunged entry requires that the entry be copied to the dirty // map and unexpunged with mu held. // 一个只读的数据结构，因为只读，所以不会有读写冲突。 // 所以从这个数据中读取总是安全的。 // 实际上，实际也会更新这个数据的entries,如果entry是未删除的(unexpunged), 并不需要加锁。如果entry已经被删除了，需要加锁，以便更新dirty数据。 read atomic.Value // readOnly
// dirty contains the portion of the map's contents that require mu to be // held. To ensure that the dirty map can be promoted to the read map quickly, // it also includes all of the non-expunged entries in the read map. // // Expunged entries are not stored in the dirty map. An expunged entry in the // clean map must be unexpunged and added to the dirty map before a new value // can be stored to it. // // If the dirty map is nil, the next write to the map will initialize it by // making a shallow copy of the clean map, omitting stale entries.
// misses counts the number of loads since the read map was last updated that // needed to lock mu to determine whether the key was present. // // Once enough misses have occurred to cover the cost of copying the dirty // map, the dirty map will be promoted to the read map (in the unamended // state) and the next store to the map will make a new dirty copy. // 当从Map中读取entry的时候，如果read中不包含这个entry,会尝试从dirty中读取，这个时候会将misses加一， // 当misses累积到 dirty的长度的时候， 就会将dirty提升为read,避免从dirty中miss太多次。因为操作dirty需要加锁。 misses int }
readOnly
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// readOnly is an immutable struct stored atomically in the Map.read field. type readOnly struct { m map[interface{}]*entry // true if the dirty map contains some key not in m. // 如果Map.dirty有些数据不在中的时候，这个值为true amended bool }
// An entry is a slot in the map corresponding to a particular key. type entry struct { // p points to the interface{} value stored for the entry. // // If p == nil, the entry has been deleted and m.dirty == nil. // // If p == expunged, the entry has been deleted, m.dirty != nil, and the entry // is missing from m.dirty. // // Otherwise, the entry is valid and recorded in m.read.m[key] and, if m.dirty // != nil, in m.dirty[key]. // // An entry can be deleted by atomic replacement with nil: when m.dirty is // next created, it will atomically replace nil with expunged and leave // m.dirty[key] unset. // // An entry's associated value can be updated by atomic replacement, provided // p != expunged. If p == expunged, an entry's associated value can be updated // only after first setting m.dirty[key] = e so that lookups using the dirty // map find the entry.
// A Value provides an atomic load and store of a consistently typed value. // Values can be created as part of other data structures. // The zero value for a Value returns nil from Load. // Once Store has been called, a Value must not be copied. // // A Value must not be copied after first use. type Value struct { noCopy noCopy
func (m *Map) Load(key interface{}) (value interface{}, ok bool) { read, _ := m.read.Load().(readOnly) e, ok := read.m[key] // 如果没找到，并且m.dirty中有新数据，需要从m.dirty查找，这个时候需要加锁 if !ok && read.amended { m.mu.Lock() // Avoid reporting a spurious miss if m.dirty got promoted while we were // blocked on m.mu. (If further loads of the same key will not miss, it's // not worth copying the dirty map for this key.) //double check,避免加锁的时候m.dirty提升为m.read,这个时候m.read可能被替换了。 read, _ = m.read.Load().(readOnly) e, ok = read.m[key] if !ok && read.amended { e, ok = m.dirty[key] // Regardless of whether the entry was present, record a miss: this key // will take the slow path until the dirty map is promoted to the read // map. m.missLocked() } m.mu.Unlock() } if !ok { return nil, false } return e.load() }
// Store sets the value for a key. func (m *Map) Store(key, value interface{}) { read, _ := m.read.Load().(readOnly) // 从 read map 中读取 key 成功并且取出的 entry 尝试存储 value 成功，直接返回 if e, ok := read.m[key]; ok && e.tryStore(&value) { return }
m.mu.Lock() read, _ = m.read.Load().(readOnly) if e, ok := read.m[key]; ok { if e.unexpungeLocked() {//确保未被标记成删除，即e 指向的是非 nil 的 // The entry was previously expunged, which implies that there is a // non-nil dirty map and this entry is not in it. //m.dirty中不存在这个键，所以加入m.dirty m.dirty[key] = e } e.storeLocked(&value) } else if e, ok := m.dirty[key]; ok { e.storeLocked(&value) } else { if !read.amended { // We're adding the first new key to the dirty map. // Make sure it is allocated and mark the read-only map as incomplete. m.dirtyLocked() m.read.Store(readOnly{m: read.m, amended: true}) } m.dirty[key] = newEntry(value) } m.mu.Unlock() }
// tryStore stores a value if the entry has not been expunged. // // If the entry is expunged, tryStore returns false and leaves the entry // unchanged. func (e *entry) tryStore(i *interface{}) bool { p := atomic.LoadPointer(&e.p) if p == expunged { return false } for { if atomic.CompareAndSwapPointer(&e.p, p, unsafe.Pointer(i)) { return true } p = atomic.LoadPointer(&e.p) if p == expunged { return false } } }
read, _ := m.read.Load().(readOnly) m.dirty = make(map[interface{}]*entry, len(read.m)) for k, e := range read.m { if !e.tryExpungeLocked() { m.dirty[k] = e } } }
func (e *entry) tryExpungeLocked() (isExpunged bool) { p := atomic.LoadPointer(&e.p) for p == nil { // 将已经删除标记为nil的数据标记为expunged if atomic.CompareAndSwapPointer(&e.p, nil, expunged) { return true } p = atomic.LoadPointer(&e.p) } return p == expunged }
// unexpungeLocked ensures that the entry is not marked as expunged. // If the entry was previously expunged, it must be added to the dirty map // before m.mu is unlocked.
// Delete deletes the value for a key. func (m *Map) Delete(key interface{}) { read, _ := m.read.Load().(readOnly) e, ok := read.m[key] if !ok && read.amended { m.mu.Lock() read, _ = m.read.Load().(readOnly) e, ok = read.m[key] if !ok && read.amended { delete(m.dirty, key) } m.mu.Unlock() } if ok { e.delete() } }
func (e *entry) delete() (hadValue bool) { for { p := atomic.LoadPointer(&e.p) // 已标记为删除 if p == nil || p == expunged { return false } // 原子操作，e.p标记为nil if atomic.CompareAndSwapPointer(&e.p, p, nil) { return true } } }
疑问
已经删除的key,再次Load的时候，会怎么样？
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func (e *entry) load() (value interface{}, ok bool) { p := atomic.LoadPointer(&e.p) if p == nil || p == expunged { return nil, false } return *(*interface{})(p), true }
